Insulin promotes human muscle protein anabolism by inhibiting proteolysis. It has been proposed that elevating the plasma amino acid concentrations would allow expression of an effect of insulin to stimulate protein synthesis. To examine the effects of elevating plasma amino acids with or without supplemental insulin on human muscle protein turnover, we will measure phenylalanin (PHE) balance (BAL) and H-PHE kinetics in both forearms of postabsorptive, healthy adults during a continuous H-PHE infusion. The studies to date indicate that very high doses of insulin are required to stimulate protein synthesis. At physiologic concentrations, insulin's major anabolic effect is to retard protein degradation.